The present invention relates to a method of producing an optical element, in particular an interferometric sensor, and to the optical element produced thereby.
More specifically, the present invention relates to a method of producing an optical element which may be used to advantage in Fabry-Perot and Fizeau interferometric sensors, which provide for high resolution temperature, density, pressure, mechanical stress, mechanical vibration, acoustic wave and magnetic field measurements, as well as for measuring the properties, e.g. thermal, electric, magnetic, chemical, mechanical, etc., of materials.
As is known, in addition to a light source and a photodetector, Fabry-Perot interferometers also comprise a transparent cavity defined by two flat parallel reflecting surfaces (at least one of which is only partially reflecting to allow the light to travel out of the cavity) and in turn defining a resonating cavity for the light emitted by the light source.
Fabry-Perot interferometers are produced in various configurations according to the type of measurement being made.
In one known configuration, the ends of a first and second optical fiber of the same type are placed facing each other to define an optical resonating cavity between the two ends. U.S. Pat. No. 5,392,117 describes an optical element for an interferometric sensor.
For the interferometer to function correctly, the ends of the two fibers must be aligned extremely accurately, must be spaced accurately to define a cavity of predetermined length, and must be locked in this position to define a stable cavity subject to no time-or temperature-induced variation in size (other than that produced deliberately for effecting the measurement). For this purpose, various solutions have been proposed.
In one known technique, the ends of the two fibers are inserted inside opposite ends of a capillary tube of an inside diameter slightly larger than the outside diameter of the fibers, and are positioned facing each other at a distance equal to the required length of the cavity. While ensuring accurate alignment of the fibers, the above technique is difficult to implement on account of the small tolerances involved. Moreover, while enabling the two fibers to be spaced according to the size of the cavity, the connection between the capillary tube and the fibers is weak, and may result in instability of the cavity due to undesired axial movement of the fibers during the measurement.
A further known technique employs a similar capillary tube inside which the end of one of the two fibers is fused; and the end of the second fiber is inserted inside the other end of the tube, is spaced as required with respect to the first fiber, and is then bonded to the tube using an epoxy resin. In this case, however, difficulty is encountered in aligning the second fiber with the first; eventual degradation and the sensitivity to high temperature of the epoxy resin eventually impair the stability of the connection; and the process as a whole is more complex than the first.
In yet a further known technique, a capillary tube of the same outside diameter as the fibers and of a length equal to the required length of the cavity is interposed between the facing ends of the two fibers; and the end edges of the capillary tube are fused with those of the adjacent fibers to form a cavity which is inaccessible from the outside. The disadvantage of this technique lies in the aligning precision involved.
Moreover, all the above techniques result in a closed cavity, thus preventing metalization of the ends of the fibers (to vary the response characteristics of the interferometer) and the insertion inside the cavity of optical elements or substances required for performing certain types of measurement.
It is an object of the present invention to provide a method of stably connecting the ends of two fibers defining a resonating cavity of an interferometer, and designed to overcome the aforementioned drawbacks.
According to the present invention, there is provided a method of producing an optical element, in particular an interferometer sensor, as claimed in claim 1.
The present invention also relates to an optical element produced using the above method.
According to the present invention, there is provided an optical element as claimed in claim 14.